At times patient, at times pushing, the cold-case detectives again went at Lloyd Welch inside the small interrogation room.

Sheila and Katherine Lyon.

It was their 11th session with the longtime sex offender, who held answers to questions that had haunted a Maryland family for more than 40 years: What happened to Katherine and Sheila — the Lyon sisters — after they vanished from a shopping mall in 1975?

“I know I should be worried about the girls, the family, puttin’ it to rest and stuff like that,” Welch told the detectives. “But you also got to look at it, I’m a survivor. I’ve lived on the street. And like I told you, I’ve also gotta think of me. What’s going to happen to me?”

But soon, Welch was describing a gruesome story. In the days after the girls were abducted, Welch said, he’d gone into a dungeonlike basement, where he saw his father and an uncle dismember one of the girls. Her remains were put into a large bag, Welch said, which was taken to rural Bedford County, Virginia, and thrown into fire.

His words, on May 12, 2015, further implicated Welch in the deaths of the Lyon sisters, to which he pleaded guilty Tuesday in Bedford, Virginia.

The conviction of the former carnival worker, after 42 years, marked an extraordinary moment in a case that stunned the region in 1975. The girls’ disappearance on a day when they had walked to a mall to have lunch, meet friends and look at Easter decorations at Wheaton Plaza became a seminal event for thousands of people, convincing them the world was no longer as safe as they had believed. That Kate, 10, and Sheila, 12, had seemed to vanish, and no culprits had been caught, enhanced the terror.

Welch’s plea to two counts of first-degree felony murder answered some but not all of the lingering questions in one of the Washington area’s most painful mysteries.

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Left unknown is who — if anyone — besides Welch was involved in the Lyon sisters’ deaths, where they were killed and where the bodies are. Authorities have said other participants in the murders are either dead or their roles could not be proven.

“It keeps me up at night,” one of the investigators said recently.

Welch, 60, stood before a judge and admitted that he participated in the abduction of the sisters when he was 18.

He did not admit to directly killing either girl but was held accountable for their deaths under a felony murder doctrine for killings “in the commission of abduction with intent to defile.”

Welch received a sentence of 48 years in an agreement with prosecutors. Given his age, and that he still must finish a prison sentence in Delaware for the unrelated sexual assault of a 10-year-old, it is unlikely that he ever will be released. Welch was prosecuted in the Lyon sisters’ case in Bedford County, some 200 miles southwest of Washington, because authorities established that the remains of at least one may have been buried there.

Welch’s recollections of the murder in the basement that he relayed to detectives shed light on why so many questions in the case linger.

His father died in 1998. The uncle has denied any involvement and after being investigated was not charged. Detectives found what they thought was human blood in the basement and even a sample of DNA, but it wasn’t of the quality to make a match.

Welch’s arrest came after Montgomery police decided about five years ago to make one final push to solve the mystery. The approach: Let’s act as if a call had just come in for the two missing girls and scour the many boxes of case records as if starting from scratch.

One of the intriguing finds was a brief report about an 18-year-old named Lloyd Lee Welch who had gone up to a security guard at the mall a week after the disappearance and said he’d been there on the day the girls went missing.

The teenage Welch reportedly said he’d seen a man talking to the sisters. Mall security called police, who administered a lie-detector test. Welch failed and was apparently dismissed by detectives at the time as an unreliable witness.

Lloyd Welch in 1977 mugshot and in the courtroom, Sept. 12, 2017.

What the detectives who were newly plowing the case discovered was that Lloyd Welch later had compiled an extensive criminal record. A 1977 home burglary case yielded a mug shot. It bore a striking resemblance to a composite sketch drawn in 1975 of a man who witnesses said stared at the Lyon girls so intently at the mall that one of the girls’ friends confronted him.

The newly assigned detectives learned that Welch was in a Delaware prison for sexually assaulting a 10-year-old girl in that state in 1997. Unsure of what to expect, they drove to see him.

Welch spoke to them — for eight hours.

He acknowledged he was at the mall the day the Lyon sisters were reported missing, according to an affidavit detectives later submitted in court. When asked what happened to the girls, Welch said he believed they were “abducted, raped and burned up,” according to the detectives’ affidavit.

As the detectives gathered Welch’s history, they repeatedly returned to interview him. He would shift his story, offering names of relatives that he said he had seen abduct the Lyon girls. He also named relatives he said he had seen abuse and kill at least one of the girls.

Finally, in their visit on May 12, 2015, detectives tried to coax from Welch, in detail, that assertion of having witnessed a murder. It was then that he gave an account — gruesome in the extreme and yet teasingly credible — of events that took place in a house in Hyattsville, Maryland. His father and stepmother lived there, he said, and it had a concrete, dungeonlike basement with access only from an external door.

His father was there, he said, and his uncle. In the end, Lloyd Welch was the one indicted.